The number of citizen’s arrests in the Metropolitan Police area has almost halved over two years, a think-tank has reported

A survey for Policy Exchange found a majority of Britons – 67% - would not act if they saw a group of teenagers drinking and verbally abusing passers-by in the street.

About 27% said they would intervene.

A freedom of information request by Policy Exchange revealed citizen's arrests went down from 3,755 in 2009-10 to 1,816 in 2011-12.

The group proposed in its report that citizen police academies be set up to train the public on how to fight crime.

"It's quite understandable that most people feel reluctant to be a 'have-a-go' hero and it is important that they have the confidence to intervene and know when it is appropriate," said the report's author, Edward Boyd.

"Citizen police academies are one way of helping the public feel more confident about their role in preventing criminal activity."

The survey found that over a third of adults would be interested in attending a free evening class at a citizen police academy.

There, police officers and volunteers would teach them how to perform a citizen's arrest and how to avoid danger when walking home alone, among other things.

The public would also like to see private businesses or social enterprises work with police in areas such as IT support and administration as a way to free up officers, the survey found.

Some 47% of those who took part in the survey said they would be willing to see private organisations policing crime scenes.